1. Field of the Invention
The present invention relates to a protection device structure, particularly to an electrostatic discharge protection device structure.
2. Description of the Related Art
A diode having a simple structure is usually used as an ESD (Electrostatic Discharge) protection device in an IC. The traditional diode ESD protection circuit is shown in FIG. 1. As to the ESD protection circuit, the more the cascaded diodes 10 are, the larger the substrate leakage current is. A parasitic PNP BJT constructed by the P+/NW diode and the P-type substrate induces the substrate leakage current. When the diodes cascaded, the PNP BJTs forms the Darlington Circuit. The more the cascaded diodes, the higher the gain of the Darlington Circuit is, and the larger the substrate leakage current is.
In order to the above-mentioned problems, The U.S. Pat. No. 6,617,650, the U.S. Pat. No. 6,972,476 and the U.S. Pat. No. 7,696,580
provide methods for improving a diode structure, which solve the problem that the substrate leakage current is induced in using the ESD protection circuit consisting of traditional diodes. However, the traditional P+/NW diode and the ESD protection circuits of the three US patents have the same disadvantage. The more the diodes of the ESD protection circuit are, the larger the impedance of discharging paths provided by the ESD protection circuit is. The ESD current no more flows to the ground terminal via the discharging paths, but flows to the ground terminal via low impedance paths of the IC. The phenomena easily results in that internal circuits of the IC is broken due to the ESD current. FIG. 2 shows an I-V curve of a TLP (Transmission Line Pulsing System) of the diode ESD protection circuit. From FIG. 2, the more the cascaded diodes are, the larger the turn-on resistance is. In other words, under the high current condition, the ESD protection circuit consisting of more diodes has a higher clamping voltage and the worse efficiency for protecting the internal circuits of the IC.
To overcome the abovementioned problems, the present invention provides an electrostatic discharge protection device structure, so as to solve the afore-mentioned problems of the prior art.